A liquid crystal panel in a liquid crystal display device includes TFTs arranged in a matrix. The TFTs are switching components for controlling operations of pixels. Silicon semiconductors such as amorphous silicon semiconductors have been used for semiconductor films of the TFTs. In recent years, use of oxide semiconductors having higher electron mobility for semiconductor films has been proposed. An example of a liquid crystal display device including TFTs using such oxide semiconductors as switching components is disclosed in Patent Document 1.